Antenna circuit for radio receivers and the like



June 4-, 194-0. J H, PRESSLEY 2Q2Q3A42 ANTENNA CIRCUIT FOR RADIO RECEIVERS AND THE LIKE Filed Oct. 28, 1938 copper or the like.

Patented June 4, 1940 PATENT OFFICE ANTENNA CIRCUIT FOR RADIO RECEIVERS AND THE LIKE Jackson H. Pressley, Grosse Pointe Park, Mich.,

assignor to Philco Radio and Television Corporation, Philadelphia, Pa., a

Delaware corporation of Application October 28, 1938, Serial No. 237,564 In Great Britain November 1,1937

8 Claims.

This invention relates to improvements in radio receivers for use in motor vehicles, such as the automobile radio receiver which is now commonly used. The principal object .of the invention is to improve the operation of such a radio receiver by increasing the amount of useful signal that is transferred from the signal collector or antenna to the loud speaker. More specifically, the invention has for its object to eliminate the signal reduction that is caused by the useof a high impedance antenna with such a radio receiver. i

. It is practically necessary to employ a low capacity, high impedance antenna inconjunction with a radio receiver for use in the modern automobile, in order to obtain satisfactory signal pick-up. The most common form of low capacity automobile radio antenna is that in the form of a rod which extends vertically upward from some part of the automobile. Such an antennais very efiicient in picking up signals but it has low capacity and high impedance compared to that of the transfer circuit and the tuned input circuit of the receiver, and therefore there is a substantial reduction in signal as explained more fully hereinafter. The present invention eliminates this defect in a very simple manner.

The invention may be understood more readily by reference to the accompanying drawing, the single figure of which is a diagrammatic illustration of. an automobile radio receiver embodying the invention.

Referring to the drawing, there is shown the radio receiver housing 5 which contains the receiver elementsincluding the tuned input transformer T, the secondary of which is loosely coupled to the primary. The other elements of the receiver play no part directly with respect to this invention, and since they are conventional elements, there is no need to illustratethem. The primary P of the transformer T may be selftuned by virtue of its distributed capacitance and the external capacityof the antenna and transfer circuit to a frequency just below the broadcast band (550 k. c. to 1500 k. 0.), which frequency is usually about 460 k. c.

A low capacity, high impedance antenna A, the capacity of which withrespect to earth may he around 17 micro-microfarads, supplies the collected signal energy to-the receiver via the lead-in conductor 2, which. is shielded throughout a portion of its length by means of the tubular shield 33 which may consist of flexible braided The capacity to earth of such a shieldedconductor is. often comparatively high and may be of the order of 100 micro-microfarads. The low capacitance of the antenna in circuit with the higher capacity of the transfer circuit causes a loss in signal strength "in the signal supplied to the input circuit, and as a result of the relative impedance values, a reduc tion of the order of 4 or 5- to 1 in the signal voltage may take place. In other words, the antenna capacity incombination with that. of the transfer circuit in effect forms a capacitance potentiometervwhich seriously reduces the amplitude of signal supplied to the tuned circuit.

In accordance with the present invention, there is provided an inductance L in the antenna circuit close to the antenna and between the antenna and the shield 3, which inductance is so designed that it forms a tuned circuit with the antenna capacity which circuit is resonant at some predetermined frequency above the band which it is desired to receive. Thus if it is desired to receive signals over the broadcast band, the elements may be designed to resonate at a frequency of approximately 1800 k. c. In the case of the specific values above mentioned, the inductance of the coil L may be approximately 400 microhenries. By the provision of the inductance L, the effect of the high impedance of the antenna upon the incoming signal is substantially neutralized at the top end of the broadcast band and considerably minimized at the low end of the band. ,At the same time, the frequency response of the receiver is made more uniform over the broadcast band since receivers generally are more sensitive, when noise is the limiting feature, at the low end of the band.

If desired, in designing the coil L, its inductance may be chosen so as to resonate at the predetermined frequency (1800 k. c., e. g.) in combination with the combined series capacitance of the antennacapacity and the lead-in capacity. However, since the latter capacity is normally very large compared to the former, the magnitude of the two capacities in series will not differ greatly fromthat of the lower capacity. Thus 17 micro-microfarads in series with 100 micromicrofarads is roughly 15 micro-microfarads.

The coil L may comprise two pie-wound sections, each comprising 65 turns of 7-41 Litzendraht wire, the two sections being universally wound (honeycomb wound) in the same direction on a paper tube having an outside diameter of 1% of an inch with a wall thickness of .03 inch. An iron core is provided for the wound sections,

In an automobile radio, the limiting feature to sensitivity is the noise generated in the first valve. Since the invention provides a means for increasing the signal deliveredto the first valve,

it eifectively increases the sensitivity of the receiver and improvements in the signal to noise ratio of the order or magnitude of siXto one have been observed in receivers employing this invention.

It will be understood, of course, that the invention is not limited to the specific illustration and the specific values herein given since the invention is capable of various embodiments Without departing from its scope.

I claim:

1. In a radio receiving system, an antenna or signal collector, an input circuit comprising a transformer having a primary winding and a tuned secondary winding, a shielded lead-in for supplying signals from said antenna to said input circuit, said antenna having low capacity to ground and said shielded lead-in having" relatively high capacity to ground, whereby the relative impedances of the antenna and shielded lead-in tend to reduce the amplitude of the signal supplied to said input circuit, and an inductance serially interposed between said antenna and said shielded lead-in and adapted to substantially neutralize the signal reduction.

2. In a radio. receiving system, an'antenna or signal collector, an input circuit comprising a transformer having a primary winding and a tuned secondary winding, a shielded lead-in for supplying signals from said antenna to said input circuit, said antenna having low capacity to ground andsaid shielded lead-inhaving relatively high capacity to ground, whereby the relative impedances of the antenna and shielded lead-in tend to reduce the amplitude of the signal supplied to said input circuit, andan inductance serially interposed between said antenna and said shielded lead-in and adapted to substantially neutralize the signal reduction, said inductance forming with said antenna capacity a tune-d circuit resonant at a predetermined frequency.

3. In a radio receiving system, an antenna or signal collector, an input circuit comprising a transformer having a primary winding and a tuned secondary winding, a' shielded lead-in for supplying signals from said antenna to said input circuit, said antenna having low capacity to ground and said shielded lead-in having relatively high capacity to ground, whereby the relative impedance of the antenna and shielded leadin tend to reduce the amplitude of the signal suppliedto said input circuit, and an inductance serially interposed between said antenna and said shielded lead-in and adapted to substantially neutralize the signal reduction, said inductance forming with said antenna capacity and the shielded lead-in capacity a tuned circuit resonant at a predetermined frequency.

. 4, An antenna system for a radio receiver comprising an aerial, a shielded lead-in for connection with a radio receiver, and a fixed'ind'uctan'ce coil connected intermediate said aerial and said shielded lead-in in series connection \m'th each.

5. A radio antenna system for mounting in a radio frequency voltage field having a high voltage radio frequency portion and a low voltage radio frequency portion, said antenna system comprising a radio signal collector, an inductance coil and a shielded lead-in, all in electrical series connection with reference to one another, with the inductance coil in such physical relation with reference to said signal collector as to be in thehigh voltage radio frequency portion of the field and with the shielded lead-in in such physical relation with reference to said signal collector as to be in the low voltage radio frequency portion of said field, and with said shielded lead-in acting to exclude undesired extraneous noises from the antenna system.

6, A radio receiver antenna system for a mobile craft where lack of space permits only an aerial materially shorter than the length 'necessary for efiicient radio wave reception,'including in combination an aerial which is so much shorter than the length of the received radio waves as connected at the other end to the radio receiver,

a single conductor connected to said other end and intermediate said inductance and a radio receiver, and means for shielding said conductor to exclude undesired noises originating within said mobile craft, with said inductance acting to position said connected portion of the aerial in a high voltage radio frequency field.

'7. In a radio receiving system, an antenna or signal'collector of small physical size, an input circuit comprising a transformer having a secondary winding tunable over a predetermined band of frequencies and a primary winding selfresonant near the low-frequency end of said band of frequencies, a shielded lead-in for supplying signals from said antenna to said input circuit, said antenna having a low capacity to ground and said shielded lead-in having relatively high capacity to ground, and an inductance serially interposed between said antenna and said shieldantenna capacity a tuned circuit resonant near the high-frequency end of said band of frequencies.

8. In a radio receiving system, an antenna or signal collector of small physical size, an input circuit comprising a transformer having a secondary winding tunable over substantially a three-to-one range of frequencies and a high inductance primary winding, a shielded lead-in for supplying signals from said antenna to said input circuit, said primary winding being selftuned by virtue of its distributed capacitance and the external capacity of saidantenna and lead-in to a frequency near the lower end of said frequency range, said antenna having a low capacity to ground and said shielded lead-in having relatively high capacity to ground, and

an inductance serially interposed between said antenna and said shielded lead-in, said inductance forming with said antenna capacity a tuned circuit resonant near the high-frequency end of said frequency range.

JACKSON H. PRESSLEY. 

